Raman spectroscopy is a known technique of identifying the molecular composition of gases and liquids. Light, such as laser light, is directed onto the substance in question and interacts with the electron cloud of the molecules of the substance to an extent determined by the molecular polarization potential of the molecule. The oscillating electrons emit light primarily at the frequency of the incident laser (“Rayleigh scattering”). A small fraction of the light emitted occurs at different frequencies (“Raman scattering”), which correspond to vibrational energies of the molecules in the substance. The shift in the emitted photon's frequency away from the excitation frequency is known as the Raman shift. The observed pattern of Raman shift frequencies is a spectral fingerprint that allows one to identify the molecular components of the substance. This identification technique may be carried out without the need for an electrical current, so that concerns over flammability are reduced. Further, this identification technique may be less prone to nonspecific responses, may allow for the identification of gases such as hydrogen and oxygen which may not be detectable by other optical analysis methods such as infrared absorption spectroscopy, and may be dependable in challenging environments such as those that feature high humidity. However, the intensity of the Raman scattering is weak, and thus larger quantities of the analyte must be present to be detected, compared to other techniques.
Surface enhanced Raman spectroscopy (SERS) is a technique that is used to enhance the relatively weak Raman effect. A substrate that includes one or several noble metals, but typically gold and/or silver, may be used to carry out the SERS process. The gold or silver may have a roughness or patterned feature on the scale of 100 nm. Here, surface plasmons of the gold or silver are excited by the excitation light to result in an increased electrical field and a stronger Raman signal. The frequencies at which the electric field is enhanced are determined by the size of the features on the gold or silver. Selecting feature sizes so that the plasmon frequencies that are resonant with laser and Raman scattering frequencies will increase the efficiency of the Raman process. The increased effect may be achieved for molecules in proximity to particular surfaces where locally intense electric fields are present due to the excitement of plasmons.
Other techniques have been proposed in order to attempt to increase the sensitivity of the Raman measurement through the SERS effect. One such design employs a double substrate approach in which the analyte is coupled to nanoparticles that are suspended in a solution, and then the nanoparticles are coupled to a surface through non-specific interactions with a surface coating. This design is limited to use with a liquid analyte as nanoparticles placed into a gas analyte matrix would not be feasible. Further, the reliance on the random configuration of the nanoparticles and the analyte does not lead to efficient enhancement.
Another technique for SERS enhancement makes use of a SERS-active surface on the inside of a glass vial. The laser that generates the excitation light is directed in a radial direction into the glass vial and thus enters the side of the glass vial. This technique interrogates a single point on the SERS surface and does not support sampling of a large surface area. Although attempts to increase the sensitivity of a SERS process have been made, there remains room for variation and improvement in the art.
Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the invention.